1. Field of the Invention
The present invention relates to a semiconductor output circuit, and more particularly, to a semiconductor output circuit which controls power supply to a load.
2. Description of the Related Art
As a semiconductor output circuit using a source follower output transistor for supplying power to a load, for example, there is a circuit disclosed in Japanese Unexamined Patent Application Publication No. Hei 03-248619, which is equivalent to Japanese Patent No. 2646786 (B2) (hereinafter, referred to as Kanamori). The above-mentioned semiconductor output circuit is basically structured by coupling as a source follower an output transistor between a power supply line and an output terminal to which a load is coupled. Further, in order to keep a nonconductive state of the output transistor irrespective of voltage fluctuation occurring in the output terminal, a shutdown transistor is coupled between a gate and a source of the output transistor.
However, with the structure as described above, because the shutdown transistor is an enhancement type, a resistor for conductive bias is required between the gate and the source thereof, and a DC current (standby current) flows through the resistor even after the output transistor enters the nonconductive state.
As a semiconductor output circuit of another structure, there is a circuit disclosed in Japanese Unexamined Patent Application Publication No. Hei 06-188710, which is equivalent to U.S. Pat. No. 5,352,932 (hereinafter, referred to as Tihanyi), and in the semiconductor output circuit, a depletion transistor is used as the shutdown transistor.
A current flows through the depletion transistor even when its gate and source are at the same potential, and thus the standby current can be limited because the resistor which is required in Kanamori becomes unnecessary, which is more favorable.
However, the inventor has noticed the following problem. In Tihanyi, in the case where the depletion transistor serving as the shutdown transistor is caused to be in a conductive state, a power supply potential is supplied to its gate, and in the case where the depletion transistor is caused to be in a nonconductive state, a ground potential is supplied to its gate. This means that, as a result, the power supply voltage is applied between the gate and the source of the depletion transistor. Accordingly, in the case where the depletion transistor is used as the shutdown transistor, it is required to use a transistor having a higher breakdown voltage, namely, a transistor having a relatively larger device area. Therefore, the inventor has sought a semiconductor output circuit not using a device having a higher breakdown voltage but using a device having a lower breakdown voltage.